HEART.pptx histology heart histology histology

heart histology

1. HEART HISTOLOGY

2. Anatomical conception The heart is a muscular pump that maintains unidirectional flow of blood. The heart contains four chambers—the right and left atria and right and left ventricles—through which blood is pumped. Valves guard the exits of the chambers, preventing backflow of blood. An interatrial septum and an interventricular septum separate the right and left sides of the heart. The right side of the heart pumps blood through pulmonary circulation. The right atrium receives blood returning from the body via the inferior and superior venae cavae , the two largest veins of the body. The right ventricle receives blood from the right atrium and pumps it to the lungs for oxygenation via the pulmonary arteries. The left side of the heart pumps blood through systemic circulation. The left atrium receives the oxygenated blood returning from the lungs via the four pulmonary veins. The left ventricle receives blood from the left atrium and pumps it into the aorta for distribution into the remainder of the body.

3. The heart contains the following: A musculature of cardiac muscle for contraction to propel the blood. A fibrous skeleton that consists of four fibrous rings surrounding the valve orifices, two fibrous trigones connecting the rings, and the membranous part of the interventricular and interatrial septa. The fibrous rings are composed of dense irregular connective tissue. They encircle the base of the two arteries, leaving the heart (aorta and pulmonary trunk) and the openings between the atria and the ventricles (right and left atrioventricular [AV] orifices). These rings provide the attachment site for the leaflets of all four valves of the heart that allow blood flow in only one direction through the openings. The membranous part of the interventricular septum is devoid of cardiac muscle; it consists of dense connective tissue that contains a short length of the atrioventricular bundle of the conducting system of the heart. The fibrous skeleton provides independent attachments for the atrial and ventricular myocardium. It also acts as an electrical insulator by preventing the free flow of electrical impulses between atria and ventricles.

4. Conducting system A conducting system for initiation and propagation of rhythmic depolarizations , which results in rhythmic cardiac muscle contractions. This system is formed by modified cardiac muscle cells (Purkinje fibers), which generate and conduct electrical impulses rapidly through the heart. In the sudden cessation of normal heart rhythm leading to abrupt cessation of blood circulation called cardiac arrest, the conducting system of the heart fails to produce or conduct electrical impulses that cause the heart to contract and supply blood to the body. Sudden cardiac arrest is a medical emergency; first-aid treatment such as cardiopulmonary resuscitation (CPR) and defibrillation (delivering a therapeutic dose of electrical energy to the heart) can improve the chances of survival. If not treated, cardiac arrest leads to sudden cardiac death. Heart rhythm pathologies associated with cardiac arrest include tachycardia (accelerated heart rhythm), fibrillation (rapid, irregular, and ineffective contractions), bradycardia (decelerated heart rhythm), and asystole (total absence of heart rhythm).

5. Coronary vasculature A coronary vasculature that consists of two coronary arteries and cardiac veins. The right and left coronary arteries provide the arterial blood supply to the heart. They originate from the initial part of the ascending aorta near the aortic valves and circle the base of the heart, with branches converging toward the apex of the heart. Venous drainage of the heart occurs via several cardiac veins, most of which drain into the coronary sinus located on the posterior surface of the heart. The coronary sinus drains into the right atrium.

6. Microanatomy of heart The wall of the heart is composed of three layers: epicardium , myocardium, and endocardium . The structural organization of the wall of the heart is continuous within the atria and ventricles. The wall of the heart is composed of three layers. From the outside to the inside, they are as follows.

7. Epicardium The epicardium , also known as the visceral layer of serous pericardium, adheres to the outer surface of the heart. It consists of a single layer of mesothelial cells and underlying connective and adipose tissue. The blood vessels and nerves that supply the heart lie in the epicardium and are surrounded by adipose tissue that cushions the heart in the pericardial cavity. The epicardium is reflected back at the great vessels entering and leaving the heart as the parietal layer of serous pericardium, which lines the inner surface of the pericardium that surrounds the heart and roots of great vessels. Thus, there is a potential space containing a minimal amount (15 to 50 ml) of serous (pericardial) fluid between the visceral and parietal layers of the serous pericardium. This space is known as the pericardial cavity; its lining consists of mesothelial cells.

8. Myocardium The myocardium, consisting of cardiac muscle, is the principal component of the heart. The myocardium of the atria is substantially thinner than that of the ventricles. The atria receive blood from the large veins and deliver it to adjacent ventricles, a process that requires relatively low pressure. The myocardium of the ventricles is substantially thicker because of the higher pressure required to pump the blood through the pulmonary and systemic circulations

9. Cardiac Muscle Cardiac muscle occurs only in the myocardium of the heart and, to a variable extent, in the roots of large vessels where they join the heart.

10. Structure of cardiac muscle fibers Intermediate in size between skeletal and smooth muscle Fibers are cylindrical, branch, and form interwoven bundles. Usually one nucleus per fiber located in the center Organelles are clustered at the poles of the nucleus. Myofilament organization into myofibrils is identical to skeletal muscle. Cross-striations and bands identical to skeletal muscle are present, but not as prominent.

11. Junctional complexes that are unique to cardiac muscle fibers Consist of specialized cell junctions and interdigitations of the sarcolemma at the ends of the fibers Contain three types of junctions Fascia adherens . Similar to zonula adherens of epithelia; serve to attach cardiac muscle fibers and anchor actin filaments of the terminal sarcomeres at the ends of the cell. Acts as a hemi-Z-line. Desmosomes. Bind ends of fibers together Gap junctions. Provide ionic coupling between fibers Intercalated discs

12. Coordination of cardiac muscle contraction Sarcomeres, myofibrils, and myofilaments are the same as skeletal muscle fibers. T-tubules are located at the level of the Z-lines, rather than at junction of A and I bands as in skeletal muscle. No triads. Sarcoplasmic reticulum is not as well developed as in skeletal muscle fibers and does not form terminal cisterns. Contraction is initiated by intracellular calcium release. Contraction can spread through the myocardium due to the presence of gap junctions that allow calcium to flow from one fiber into another.

13. Endocardium The endocardium consists of an inner layer of endothelium and subendothelial connective tissue, a middle layer of connective tissue and smooth muscle cells, and a deeper layer of connective tissue, which is also called the subendocardial layer. The latter is continuous with the connective tissue of the myocardium. The conducting system of the heart is located in the subendocardial layer of the endocardium .

14. Septum The interventricular septum is the wall between the right and left ventricles. It contains cardiac muscle in all but the membranous portion. Endocardium lines each surface of the interventricular septum. The interatrial septum is much thinner than the interventricular septum. Except in certain localized areas that contain fibrous tissue, it has a center layer of cardiac muscle and a lining of endocardium facing each chamber.

15. Valves Heart valves are composed of connective tissue with overlying endocardium . The heart valves attach to the complex framework of dense irregular connective tissue that forms the fibrous rings and surrounds the orifices containing the valves. Each valve is composed of three layers.

16. Valves The fibrosa forms the core of the valve and contains fibrous extensions from the dense irregular connective tissue of the skeletal rings of the heart. The spongiosa is loose connective tissue located on the atrial or blood vessel side of each valve. It is composed of loosely arranged collagen and elastic fibers infiltrated with large numbers of proteoglycans . The spongiosa acts as a shock absorber to dampen vibrations associated with the closing of the valve. It also confers flexibility and plasticity to the valve cusps. In the aortic and pulmonary valves, spongiosa located on the blood vessel side is called arterialis . It corresponds to the loose connective tissue located on the atrial side of the AV (tricuspid and mitral) valves, which is called the auricularis .

17. Valves The ventricularis is immediately adjacent to the ventricular or atrial surface of each valve and is covered with endothelium. It contains dense connective tissue with many layers of elastic fibers. In the AV valves, the ventricularis continues into the chordae tendineae , which are fibrous, thread-like cords also covered with endothelium. They extend from the free edge of the AV valves to muscular projections from the wall of the ventricles, which are called papillary muscles. Valve cusps are normally avascular . Small blood vessels and smooth muscle can be found only in the base of the cusp. The surfaces of the valve are exposed to blood, and the cusps are thin enough to allow nutrients and oxygen to diffuse from the blood.

18. Intrinsic Regulation of Heart Rate Contraction of the heart is synchronized by specialized cardiac conducting cells. Cardiac muscle can contract in a rhythmic manner without any direct stimulus from the nervous system. For the heart to be an effective pump, it is necessary for the atria and ventricles to contract in a coordinated rhythmic manner. The electrical activity (impulses) that results in the rhythmic pulsations of the heart is initiated and propagated by the conducting system of the heart. The rate of depolarization of cardiac muscle varies in different parts of the conducting system; the fastest is in the atria, the slowest in the ventricles. The contraction cycle of the heart is initiated in the atria, forcing blood into the ventricles. A wave of contraction in the ventricles then begins at the apex of the heart, forcing blood from the heart into the aorta and pulmonary trunk.

19. Intrinsic Regulation of Heart Rate The conducting system of the heart consists of two nodes—the sinoatrial (or sinu-atrial ) node and the atrioventricular node—and a series of conduction fibers or bundles (tracts). Electrical impulses are generated at the sinoatrial (SA) node, a group of specialized nodal cardiac muscle cells located near the junction of the superior vena cava and the right atrium. Since the SA node has the fastest rate of depolarizations , it is referred to as the pacemaker of the heart. The pacemaker rate of the SA node is about 60 to 100 beats per minute. The SA node initiates an impulse that spreads along the cardiac muscle fibers of the atria and along internodal tracts composed of modified cardiac muscle fibers.

20. Intrinsic Regulation of Heart Rate The impulse is then picked up at the atrioventricular (AV) node and carried across the fibrous skeleton to the ventricles by the AV bundle (of His). The bundle divides into smaller right and left bundle branches and then into subendothelial branches, commonly called Purkinje fibers. The components of the conducting system convey impulses at a rate approximately four times faster than the cardiac muscle fibers and are the only elements that can convey impulses across the fibrous skeleton.

21. Intrinsic Regulation of Heart Rate The nodal cardiac muscle cells in both the SA and AV nodes are modified cardiac muscle fibers that are smaller than the surrounding atrial cardiac muscle cells. They contain fewer myofibrils and lack typical intercalated discs. The AV bundle, the bundle branches, and the Purkinje fibers are also composed of modified cardiac muscle cells, but they are larger than the surrounding ventricular muscle cells.

22. Intrinsic Regulation of Heart Rate The terminal ramifications of the conducting system consist of Purkinje fibers. Cardiac conducting cells that make up the bundle of His originate at the AV node, pass through the fibrous skeleton of the heart, course along both sides of the interventricular septum, and terminate as Purkinje fibers in the myocardium of the ventricles. The cells that form the Purkinje fibers are larger than ventricular muscle cells. Their myofibrils are located at the periphery of the cell. The nuclei are round and are larger than the nuclei of the cardiac muscle cells in the myocardium. Because of the considerable size of the cells, the nuclei are often not included in the section. Intercalated disks are present in Purkinje fibers, but they are variable in appearance and number depending on their location. They are positive for periodic acid-Schiff (PAS) staining because of the large amount of glycogen they contain. With hematoxylin and eosin (H&E) and most other stains, the glycogen-rich center portion of the cell appears homogeneous and stains pale. Because of the stored glycogen, Purkinje fiber cells are more resistant to hypoxia than are ventricular muscle cells.

23. Systemic Regulation of Heart Function As mentioned above, the heart beats independently of any nervous stimulation. This spontaneous rhythm of the heart can be altered by nerve impulses from both sympathetic and parasympathetic divisions of the autonomic nervous system. The autonomic nerves do not initiate contraction of the cardiac muscle but rather regulate the heart rate (a chronotropic effect) according to the body’s immediate needs. Stimulation of the parasympathetic nerves decreases the heart rate.

24. Systemic Regulation of Heart Function The heart rate and the force of contraction can be regulated by circulating hormones and other substances. Changes in the force and rate of cardiac muscle contractions are regulated by hormones secreted from the adrenal medulla. These hormones include epinephrine and norepinephrine that reach the heart muscle cells via the coronary circulation.

25. Systemic Regulation of Heart Function The central nervous system monitors arterial pressure and heart function through specialized receptors located within the cardiovascular system. The activity of the cardiovascular system is monitored by specialized centers in the central nervous system (CNS). Specialized sensory nerve receptors that supply afferent information about blood pressure are located in the walls of large blood vessels near the heart and within the heart itself. The information received from all types of cardiovascular receptors initiates the appropriate physiologic reflexes. The receptors function as follows.

26. Systemic Regulation of Heart Function Baroreceptors (high-pressure receptors) sense arterial blood pressure. These receptors are located in the carotid sinus and aortic arch. Volume receptors (low-pressure receptors) located within the walls of the atria and ventricles sense central venous pressure and provide the CNS with information about cardiac distention. Chemoreceptors detect alterations in oxygen, carbon dioxide tension, and pH. These receptors are the carotid and aortic bodies located at the bifurcation of the common carotid arteries and in the aortic arch, respectively.

27. Systemic Regulation of Heart Function The carotid bodies consist of cords and irregular groups of epithelioid cells. A rich supply of nerve fibers is associated with these cells. The neural elements are both afferent and efferent. The structure of the aortic bodies is essentially similar to that of the carotid bodies. Both receptors function in neural reflexes that adjust cardiac output and respiratory rate.